European Patent 0 076 683 describes a system which serves to control the pH value of a continuous stream of liquid and in which a pH value analyzer controls the supply of acid or alkaline solution to the liquid in order to maintain a constant pH value.
This known process cannot be used, however, for control of a pH value at such low levels because it does not permit a continual measurement of the pH value of the acid scrubbing liquid for a long time without damage to the sensing element and hence at a low cost.